The present invention relates to a cellulose compound hollow fiber membrane suitable in particular for water treatment in natural water areas such as rivers, lakes and marshes.
Cellulose acetate membranes have been used as asymmetrical reverse osmosis membranes for converting sea water into fresh-water and blood dialysis membranes, and cellulose acetate membranes of hollow fiber membrane directed to these uses have been developed (e.g., JP-A 54-88881, JP-A 61-185305, JP-A 60-29763, JP-A 63-17922, JP-A 5-228208 and JP-A 6-343842). Further, cellulose compound or cellulose ester membranes other than the cellulose acetate membranes have also been studied to improve biological compatibility and water permeability of the cellulose acetate membranes. For example, cellulose compound membranes to which cellulose acetate propionate was applied are disclosed in JP-A 57-133211, JP-A 60-43442, JP-A 60-5202, JP-A 62-290468, JP-A 1-20245, JP-A 2-12611, JP-A 2-211228, JP-A 6-277473 and JP-A 6-31144.
However, none of these prior art specifically describe a hollow fiber membrane exhibiting both a high water permeation rate and high strength. Further, the membranes disclosed in these prior art are dialysis membranes or pervaporization membranes which are also membranes with a small pore diameter or non-porous membranes, and these are different from membranes with high fractionation performance used for water treatment.
Cellulose acetate hollow fiber membranes used for the purpose of water treatment are disclosed in e.g. JP-A 6-343842 and JP-A 8-108053, but it is problematic that when these cellulose acetate hollow fiber membranes are used in purification of natural water such as river water, underground water, lake water, marsh water and seawater, the membranes are degraded by microorganisms in the raw water. Accordingly, in purification of natural water by the cellulose acetate hollow fiber membrane, sterilization with sodium hypochlorite is carried out constantly or intermittently to prevent the microbial degradation of the membrane. However, when sodium hypochlorite binds to humus in natural water, there arises the problem of generation of sterilization byproducts such as carcinogenic trihalomethane.
The present invention can solve the problem that the cellulose acetate membrane is liable to microbial degradation as described above. The present invention is applicabale to purification of natural water. The invention can be conducted without sterilization treatment by an aqueous solution of sodium hypochlorite etc., with reduced frequency of the treatment. It is not involved in microbial degradation. The present invention provides a cellulose compound hollow fiber membrane with a high water permeation rate and an excellent mechanical strength.
The present invention provides a hollow fiber membrane comprising a cellulose compound as a membrane material, wherein the thickness of the membrane is 50 to 500 xcexcm, a dense membrane surface having an average pore diameter of 0.001 to 0.05 xcexcm exists on the internal and/or external surface thereof, the inside of the membrane is composed of a three-dimensional network-like porous structure having voids with an average pore diameter of 0.05 to 2 xcexcm, and 70% by weight or more of the membrane material is cellulose acetate propionate or cellulose acetate butyrate.
The hollow fiber of the present invention is used as permeation membranes for purification of water, natural water etc., and treatment of waste water. The present invention provides a method for purifying water by use of the above-mentioned hollow fiber as a permeation membrane and use of the above-mentioned hollow fiber for a permeation membrane. Permeation may be preferably conducted at the pure water permeation rate of 200 1/(m2.h) or more at a transmembrane pressure of 100 kPa and at a temperature of 25xc2x0 C.
The invention provides permeation of water or filtration of matters to be removed out by use of the above-mentioned hollow fiber membrane. It may be used for permeation, osmosis or reverse osmosis.
The cellulose compound hollow fiber membrane of the present invention (referred to hereinafter as xe2x80x9chollow fiber membranexe2x80x9d) has a membrane thickness of 50 to 500 xcexcm, preferably 100 to 400 xcexcm, in order to confer a good balance of mechanical strength and water permeability thereon.
The internal and/or external surface of the hollow fiber membrane of the present invention has a dense membrane surface having an average pore diameter of 0.001 to 0.05 xcexcm, preferably 0.005 to 0.03 xcexcm. The above-mentioned range of the average pore diameter corresponds, in terms of molecular weight cut-off, to 10,000 to 500,000, preferably 70,000 to 300,000. The internal and/or external surface of the hollow fiber membrane of the present invention is a dense membrane surface as described above, and the one wherein both the internal and external surfaces are dense membrane surfaces is particularly preferable.
The inside of the hollow fiber membrane of the present invention is composed of a three-dimensional network-like porous structure having voids with an average pore diameter of 0.05 to 2 xcexcm, preferably 0.1 to 1 xcexcm. This xe2x80x9cthree-dimensional network-like porous structurexe2x80x9d provides the hollow fiber membrane with a good mechanical strength and a good extensibility. It has voids having a larger average pore diameter than the dense surface layer. It is preferable that the inner structure does not contain plural gigantic voids having a pore diameter of 10 xcexcm or more, but it is more preferably free of such gigantic voids having a pore diameter of 10 xcexcm or more.
The average pore diameter of the inside of the membrane can be determined by taking electron microphotographs (at a magnification xc3x9710,000) of 10 sections of the membrane at equal intervals from the internal surface to the external surface of the membrane, calculating the average diameter of pores being present in 5 xcexcm2 of each section of the microphotographs and further calculating the average of the 10 average diameters. In the relationship between distances in the thickness direction of the membrane and the average pore diameter of each section, that is, in the distribution of pore diameters in the three-dimensional network-like porous structure in the inside of the membrane, there is at least one maximum value, and the maximum value (maximum pore diameter) is 0.5 to 5 xcexcm, preferably 1 to 4 xcexcm.
The hollow fiber membrane of the present invention comprises a cellulose compound as a membrane material, and 70% or more, preferably 80% by weight or more of the membrane material is cellulose acetate propionate or cellulose acetate butyrate, and more preferably 100% by weight of the membrane material is cellulose acetate propionate or cellulose acetate butyrate. When the content of cellulose acetate propionate or cellulose acetate butyrate is 70% by weight or more, the resulting membrane is hardly degraded by microorganisms and is excellent in compatibility even if mixed with other cellulose compounds, and a reduction in the mechanical strength of the membrane can thus be prevented.
The degree of substitution of acetyl and propionyl in cellulose acetate propionate and the degree of substitution of acetyl and butyryl in cellulose acetate butyrate, though being not particularly limited, are preferably 1.5 to 2.9, more preferably 2.0 to 2.8. Further, the ratio of acetyl to propionyl or of acetyl to butyryl is not particularly limited. Further, the number average molecular weight of cellulose acetate propionate and cellulose acetate butyrate is preferably 10,000 to 500,000, more preferably 50,000 to 200,000, to improve spinning properties.
When a mixture consisting of cellulose acetate propionate or cellulose acetate butyrate and other cellulose compounds is used as a material of the hollow fiber membrane of the present invention, the other cellulose compounds may be cellulose ester compounds such as cellulose diacetate, cellulose triacetate and cellulose butyrate, and cellulose ether compounds such as methyl cellulose and ethyl cellulose, and further, polysulfone polymers, polyacrylonitrile polymers, polyamide polymers, polyvinyl pyrrolidone, polyvinyl formal etc. can be also used in combination therewith.
In the hollow fiber membrane of the present invention, the tensile strength at break is preferably 3 MPa or more, more preferably 4 MPa or more, and the tensile elongation at break is preferably 15% or more, more preferably 20% or more, to maintain durability for a prolonged period of time.
In the hollow fiber membrane of the present invention, the rate of pure water permeation rate at a transmembrane pressure of 100 kPa at a temperature of 25xc2x0 C. is preferably 200 L/(m2.h) or more, more preferably 300 L/(m2.h) or more. Herein, xe2x80x9cpure waterxe2x80x9d refers to water obtained by filtering deionized water having an electrical resistance of 0.2 xcexcS/cm or less through a membrane having a molecular weight cut-off of 30,000.
The cellulose derivative hollow fiber membrane according to the present invention can be produced by e.g. phase conversion methods in wet or dry wet processes.
A spinning stock solution used in these methods is obtained by dissolving 70% by weight or more of cellulose compounds including cellulose acetate propionate or cellulose acetate butyrate as the membrane material in a solvent as described above.
This solvent may be any solvent which can be mixed with water and includes acetone, dioxane, acetic acid, dimethyl sulfoxide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethylformamide etc. Among these, high-boiling solvents capable of spinning at 100xc2x0 C. or more, for example dimethyl sulfoxide, dimethylacetamide and N-methyl-2-pyrrolidone are preferable to increase the water permeation rate, and dimethyl sulfoxide is preferable for forming the prescribed three-dimensional network-like porous structure to improve mechanical strength.
In addition to these solvents, it is further possible to add non-solvents such as ethylene glycol and polyethylene glycol and metal compounds such as lithium chloride, calcium chloride, magnesium chloride, lithium nitrate, barium nitrate, magnesium nitrate, lithium acetate and magnesium acetate.
The concentration of cellulose compounds in the spinning stock solution is preferably 15 to 35% by weight, more preferably 20 to 30% by weight.
An inside coagulating solution or a coagulation water bath used in production include non-solvents for cellulose compounds, that is, water, ethylene glycol, polyethylene glycol or a mixture of these non-solvents and the organic solvents described above. Among these, the inside coagulating solution is preferably water or a combination of water and polyethylene glycol, and the coagulation water bath is preferably water or a combination of water and the organic solvent used in the spinning stock solution.
The temperature of the inside coagulating solution and the coagulation water bath is preferably 30 to 90xc2x0 C. At a temperature of 30xc2x0 C. or more, the thickness of the dense layer as the surface of the membrane can be suitably adjusted to attain a high water permeation rate, while at a temperature of 90xc2x0 C. or less, the boiling of the inside coagulating liquid or the coagulation water bath does not occur thus facilitating the manufacturing procedure.
The temperature of the orifice of the nozzle for discharging the spinning stock solution is preferably 30 to 130xc2x0 C., more preferable 50 to 100xc2x0 C. In the case of spinning in a dry wet process, the distance between the discharge orifice of the nozzle and the surface of the coagulation water bath is set in the range of preferably 1 to 50 cm, more preferably 5 to 30 cm, such that the hollow fiber membrane after being discharged can remain in the air for 0.2 second or more.
The hollow fiber membrane of the present invention is suitable particularly for purification of natural water such as river water, underground water, lake water, marsh water and seawater, and can further be applied to treatment of various kinds of waste water from factories, homes etc.
As described above, the cellulose compound hollow fiber membrane according to the present invention has high mechanical strength and is hardly degraded by microorganisms so that even if washing with a germicide such as sodium hypochlorite is not conducted at all or the frequency of such treatment is considerably reduced, its high water permeability can be maintained.